1. Field of the Invention
The present invention relates to a device of evaluating magnetic read head that includes a magnetoresistive element and to a method of evaluating the magnetic read head.
2. Description of the Related Art
A thin film magnetic head, which includes an MR element exhibiting magnetoresistive (MR: Magneto-resistive) effect, is widely used in the past for reading out information written on magnetic recording media such as a hard disk. Recently, a thin film magnetic head utilizing a giant magnetoresistive element (GMR element) that exhibits giant magnetoresistive (GMR: Giant Magneto-resistive) effect is used more generally because of increasing recording density of magnetic recording media. Examples of such GMR element include a spin valve (SV: spin valve) GMR element.
The SV-GMR element has a structure in which a magnetic layer whose magnetization direction is fixed in a given direction (magnetically pinned layer), and a magnetic layer whose magnetization direction is varied in accordance with a signal magnetic field applied from outside (magnetically free layer), are stacked via a nonmagnetic interlayer. In particular, those configured to allow read currents to flow in a direction along the stacked-layer planes of the element during reading operation are called CIP (Current in Plane)-GMR element. Further, a thin film magnetic head including the same is called CIP-GMR head. In this case, electric resistance (namely, voltage) is varied when the read current is applied in accordance with a relative angle between the magnetization directions in the two magnetic layers (the magnetically pinned layer and the magnetically free layer).
Recently, to deal with further improvement in the recording density, CPP-GMR head, which includes a CPP (Current Perpendicular to the Plane)-GMR element in which read currents flow in a direction orthogonal to the stacked-layer planes at the time of reading operation, has been developed. Such CPP-GMR head generally includes a GMR element, a pair of magnetic bias layers that are arranged to face each other in a direction corresponding to a track-width direction with the GMR element in between via an insulating layer, and an upper electrode and a lower electrode that are formed to sandwich the GMR element and the pair of magnetic bias layers in between along a stacking direction. The upper and lower electrodes also serve as top and bottom shielding layers. Such CPP-GMR head recognizes an advantage in that higher output is obtainable even when reducing the dimension of element in the track-width direction, as compared with the CIP-GMR head. Namely, in the CIP-GMR head, since read currents flow in the in-plane direction, dimensional reduction in the track-width direction results in the narrowness of magnetic sensitive area through which read currents pass, thereby decreasing the amount of change in voltage. To the contrary, since read currents pass through the CPP-GMR head in the stacking direction, the dimensional reduction in the track-width direction less affects the amount of voltage changes. For this reason, the CPP-GMR head is advantageous compared with the CIP-GMR head from the standpoint of the reduction of track density, represented by the number of tracks per inch (TPI: Tracks Per Inch). What is more, since insulating layers do not have to be provided between the CPP-GMR element and the respective top/bottom shielding layers, the reduction, by the thickness thereof, of the linear recording density represented by BPI (bit per inch) is possible, as compared with the CIP-GMR head.
There is also a tunnel MR element (TMR element) that is configured to read currents flow in the direction orthogonal to the stacked-layer planes, as in the CPP-GMR element. The TMR element includes an ultra-thin insulating layer called a tunnel barrier layer, and much higher resistance change ratio than that of the above-mentioned CPP-GMR element is obtainable. For this reason, a thin film magnetic head including the TMR element (TMR head) is highly expected to be capable of dealing with a further improvement in recording density.
Though the thin film magnetic head including such a CIP-GMR element, a CPP-GMR element or a TMR element is suitable for reading from the magnetic recording medium on which high density recording is performed, instability of reading performance caused by what is called a Barkhausen noise is yet worried. The Barkhausen noise is a discontinuous noise produced when a magnetic wall of the magnetically free layer made of soft ferromagnetic material moves discontinuously due to structural defect inside the magnetically free layer or the like. The thin film magnetic head producing such a Barkhausen noise lacks reliability in its reading operation. For this reason, in general, magnetic bias layers (magnetic domain controlling layers) are arranged on both neighboring sides of the magnetically free layer along the track-width direction, for the purpose of suppressing the Barkhausen noise, as stated above. The magnetic bias layer is made of a permanent magnet etc., and functions to promote single domainization of the magnetically free layer by applying a longitudinal bias field to the magnetically free layer in the track-width direction.
As mentioned above, a predetermined longitudinal bias field needs to be applied to the magnetically free layer to suppress the Barkhausen noise. For this reason, precise evaluation of the magnetic characteristics (such as coercive force, squareness ratio) of the magnetic bias layer in a thin film magnetic head is required. Accordingly, in the past, the change in the resistance value is measured while applying an external magnetic field to the thin film magnetic head in the application direction of the longitudinal bias field (track-width direction), as can be seen in the Japanese Patent No. 3877386 gazette, for example. In this case, since the external magnetic field and the longitudinal bias field are balanced with each other at a minimum (or maximum) resistance value, the external magnetic field at that time is regarded as the coercive force of the magnetic bias layer.